This application is based on and incorporates herein by reference Japanese Patent Application Nos. 2000-126281 filed on Apr. 21, 2000, 2000-179359 file on Jun. 9, 2000, 2000-404671 filed on Dec. 28, 2000, 2000-404672 filed on Dec. 28, 2000, and 2000-404694 filed on Dec. 28, 2000.
The present invention relates to a control apparatus for an internal combustion engine, for feedback controlling an input of a subject to be controlled in an internal combustion engine.
2. Description of Related Art
In a vehicle under advanced electronic control in recent years, various controls are performed by feedback controls. For example, the feedback control is used for A/F ratio control (fuel injection control), variable valve timing control, electronic throttle control, fuel pump control, boost pressure control of a turbo charger, idle speed control, cruise control, and the like.
A conventional feedback control is carried out in such a manner that an output (controlled variable) of a subject to be controlled is detected by a sensor or the like, a correction amount of an input (operation amount) of the control subject is calculated in accordance with a deviation between the output of the control subject and a target value so that the output of the control subject coincides with the target value, and the input of the control subject is corrected by the correction amount to make the output of the control subject follow the target value.
In many cases, a system as a subject of the feedback control in a vehicle has a long waste time (a large delay element) and, moreover, the waste time varies according to the engine operating conditions, deterioration with time in a control system, and the like. Consequently, the conventional feedback control is easily influenced by the variations in waste time. When a higher gain is set to increase the response, the feedback control becomes unstable, and there is the possibility that hunting occurs. In the conventional feedback control, it is therefore difficult to realize both higher gain (higher response) and stability. Moreover, there is a drawback such that the stability is apt to deteriorate due to an influence of an error in modeling of the control subject, and robustness is low.
A vehicle has a three-way catalyst in its exhaust pipe to treat exhaust gases. In order to increase catalytic conversion efficiency, it is necessary to control the concentration of an exhaust gas to be within a catalytic conversion window (about target A/F ratio). An exhaust gas sensor (A/F ratio sensor or oxygen sensor) is disposed on each of the upstream and downstream sides of a catalyst, a fuel injection amount is feedback controlled so that the A/F ratio of an exhaust gas detected by the exhaust gas sensor on the upstream side is equal to an upstream-side target A/F ratio, and a sub-feedback control is performed to correct the upstream-side target A/F ratio so that the A/F ratio of the exhaust gas detected by the downstream-side exhaust gas sensor is equal to a downstream-side target A/F ratio.
The conventional sub-feedback control is performed by PID control. Recently, in order to increase control accuracy, as shown by the publication of JP-A-9-273439, a technique of using sliding mode control has been proposed. The sliding mode control relates to a feedback control method of a variable structure type of preliminarily building a hyperplane expressed by a linear function using a plurality of state variables of a subject to be controlled as variables, allowing a state variable to converge on the hyperplane by high gain control at high speed, and allowing the state variable to converge on a required equilibrium point on the hyperplane by an equivalent control input while restricting the state variable on the hyperplane.
Generally, the sliding mode control has an advantage that once the state variable of the control subject converges on the hyperplane, the state variable can stably converge on an equilibrium point on the hyperplane without almost no influence of disturbance or the like. However, only a mode of a subject to be controlled in the case where a state variable converges on a hyperplane is considered. Consequently, when the sliding mode control is applied to control the A/F ratio of exhaust gas as in the publication, generally, at a high gain, hunting occurs due to disturbances and waste time around the hyperplane, and a state such that the state variable does not converge on the hyperplane occurs. As shown in FIG. 25, an inconvenience such that an output of the downstream-side exhaust gas sensor (A/F ratio of the exhaust gas on the downstream side of the catalyst) does not converge on a target value (target A/F ratio on the downstream side) may occur depending on the initial states. On the other hand, at a low gain, there is a drawback such that an input is insufficient for an error in modeling, so that response deteriorates and, as shown in FIG. 26, the speed of convergence of an output of the downstream-side exhaust gas sensor (concentration of the exhaust gas on the downstream side of the catalyst) becomes conspicuously slow.
Further, as disclosed in Japanese Patent No. 2,518,247, it is proposed to increase an update amount of an exhaust gas A/F ratio feedback control constant (for example, a skip amount) as the deviation between an A/F ratio detected by the downstream-side exhaust gas sensor and the downstream-side target exhaust gas A/F ratio becomes larger.
Here, dynamic characteristics of a catalyst vary according to the degree of deterioration of the catalyst, catalytic conversion state, and engine operating conditions. However, it cannot be the that the response of sub feedback control of the conventional main/sub feedback system to a change in dynamic characteristics of a catalyst is sufficient. Consequently, there is the possibility that a delay occurs in the response of the sub feedback control to a change in dynamic characteristics of the catalyst, concentration of exhaust gas on the downstream side of the catalyst (output of the downstream-side exhaust gas sensor) becomes unstable, and hunting occurs.
A conventional feedback control is carried out in such a manner that an output (controlled variable) of a subject to be controlled is detected by a sensor or the like, a correction amount of an input (operation amount) of the control subject is calculated by proportional integral and derivative control (PID control) in accordance with a deviation between the output of the control subject and a target value so that the output of the control subject coincides with the target value, and the input of the control subject is corrected by the correction amount to make the output of the control subject follow the target value.
A correction amount calculated by a conventional feedback control using the PID control is derived by adding a proportional term, an integral term, and a differential term. Generally, in order to improve a start-up characteristic in the case where an output of a subject to be controlled follows a target value, it is effective to increase the gain of the differential term. It is presumed that, when the gain of the differential term is set to be too high, an influence of noise becomes large, overshoot occurs, and the performance of following the target value deteriorates. In the conventional feedback control, therefore, the gain of the differential term is set to be low and the gain of the proportional term is set to be high, thereby improving the performance of following the target value.
In various feedback controls regarding the engine control of a vehicle, however, a relatively large waste time and a phase delay exist in a subject to be controlled, and disturbance is large. Consequently, when the gain is increased to make response faster, the feedback control becomes unstable, and there is the possibility that hunting occurs. In the conventional feedback control, it is therefore difficult to realize both higher gain (higher response) and stability. Moreover, there is a drawback such that the stability is apt to deteriorate due to an influence of an error in modeling of the control subject, and robustness is low.
As an engine control system of a vehicle, in order to improve exhaust gas conversion efficiency of a three-way catalyst by increasing control accuracy of exhaust gas A/F ratio, there is what is called a two-sensor type exhaust gas A/F ratio control system in which a sensor for detecting A/F ratio of an exhaust gas (oxygen sensor or broad-range exhaust gas A/F ratio sensor) is disposed on each of the upstream and downstream sides of a catalyst, and which performs feedback control to make an actual exhaust gas A/F ratio on the upstream side of the catalyst coincide with a target exhaust gas A/F ratio on the basis of an output of the upstream-side sensor while carrying out sub feedback control for correcting a target exhaust gas A/F ratio of A/F ratio feedback control on the upstream side of the catalyst on the basis of an output of the downstream side sensor.
In such a two-sensor type exhaust gas A/F ratio control system, it is known that in a state where the target exhaust gas A/F ratio on the upstream side of the catalyst is deviated from a theoretical exhaust gas A/F ratio range, when the sub feedback control based on the output of the downstream side sensor is continued under conditions similar to those of the state where the target exhaust gas A/F ratio is in the theoretical exhaust gas A/F ratio range, the exhaust gas A/F ratio cannot be controlled accurately (refer to JP-A-10-30478). Specifically, when the state where the target exhaust gas A/F ratio on the upstream side of the catalyst is deviated from the theoretical exhaust gas A/F ratio continues for a while, there is a case that a harmful component adsorbing state of the catalyst becomes almost saturated. In such a state, when the sub feedback control based on the output of the downstream side sensor is continued under conditions similar to those in the state where the target exhaust gas A/F ratio is in the theoretical exhaust gas A/F ratio range (the state where the catalyst is not saturated), the target exhaust gas A/F ratio on the upstream side of the catalyst is excessively corrected. Even when the exhaust gas A/F ratio on the upstream side of the catalyst is returned to the theoretical exhaust gas A/F ratio range, a delay in the exhaust gas A/F ratio downstream of the catalyst becomes large by a substance adsorbed by the catalyst, and a return from the excessive correcting state to a normal state is delayed.
JP-A-10-30478 therefore discloses a technique of inhibiting the sub feedback control based on the output of the downstream side sensor when the target exhaust gas A/F ratio at the upstream of the catalyst is deviated from the theoretical exhaust gas A/F ratio.
When the sub feedback control based on the output of the downstream side sensor is inhibited and the exhaust gas A/F ratio feedback control is performed by using only the output of the upstream side sensor in the case where the target exhaust gas A/F ratio at the upstream of the catalyst is deviated from the theoretical exhaust gas A/F ratio, a converting state of the exhaust gas passing through the catalyst (A/F ratio of the exhaust gas downstream of the catalyst) cannot be reflected in the exhaust gas A/F ratio feedback control at all. Consequently, there is a case that the catalytic conversion efficiency deteriorates.
A first object of the present invention is to provide a control apparatus for an internal combustion engine, capable of realizing both higher gain (higher response) and stability of a feedback control and also increased robustness.
According to a first aspect of the present invention, a control apparatus for an internal combustion engine of the invention sets an intermediate target value on the basis of an output of a subject to be controlled and a final target value by intermediate target value setting means, and calculates a correction amount of an input of the subject to be controlled on the basis of the output of the subject to be controlled and the intermediate target value. By setting not only the final target value but also the intermediate target value as described above, the control is not easily influenced by variations in waste time (lag element) of the subject to be controlled and an error in modeling. While maintaining the stability of the feedback control, higher gain (higher response) can be achieved. Thus, both higher gain and stability of the feedback control can be realized, and robustness can be also increased.
A second object of the present invention is to provide an exhaust gas A/F ratio control apparatus for an internal combustion engine having improved transient characteristics during a period in which exhaust gas A/F ratio detected by a downstream-side exhaust gas sensor (A/F ratio of exhaust gas on the downstream side of a catalyst) converges to target A/F ratio and capable of realizing both prevention of hunting and improved response.
According to a second aspect of the present invention, an exhaust gas A/F ratio control apparatus for an internal combustion engine calculates a correction amount of an upstream-side target exhaust gas A/F ratio on the basis of a state variable derived from an exhaust gas A/F ratio detected by a downstream-side exhaust gas sensor by using a back stepping method. In the back stepping method, an almost ideal convergence locus of the state variable (target convergence locus) is set by a virtual input term. While converging the deviation between the state variable and the virtual input term, a control is performed in consideration of the deviation between the state variable and the target value as well. Consequently, even under the conditions that the deviation between the state variable and the virtual input term is not equal to zero, the state variable can be stably converged. Therefore, even under the conditions that an influence of disturbance and waste time is exerted and the state variable is not easily converged by the conventional sliding mode control, the state variable can be smoothly converged, and the A/F ratio of the exhaust gas on the downstream side of the catalyst can be converted to the target A/F ratio with high response.
A third object of the present invention is to provide an exhaust gas A/F ratio control apparatus for an internal combustion engine, capable of performing stable exhaust gas A/F ratio control with improved response of sub feedback control to a change in dynamic characteristics of a catalyst.
According to a third aspect of the present invention, in an exhaust gas A/F ratio control apparatus for an internal combustion engine of the invention, exhaust gas sensors are provided on the upstream and downstream sides of a catalyst, a fuel injection amount is feedback-controlled by exhaust gas A/F ratio feedback control means so that the exhaust gas A/F ratio detected by the upstream-side exhaust gas sensor becomes an upstream-side target exhaust gas A/F ratio, and the upstream-side target exhaust gas A/F ratio is corrected by sub feedback control means so that the exhaust gas A/F ratio detected by the downstream-side exhaust gas sensor becomes the downstream-side target exhaust gas A/F ratio. In the apparatus, intermediate target value setting means sets an intermediate target value of the sub feedback control on the basis of the exhaust gas A/F ratio detected by the downstream-side exhaust gas sensor and a final downstream-side target exhaust gas A/F ratio, and a correction amount of the upstream side target exhaust gas A/F ratio is calculated on the basis of the exhaust gas A/F ratio detected by the downstream-side exhaust gas sensor and the intermediate target value. In such a manner, the response of the sub feedback control to a change in dynamic characteristics of the catalyst is improved. The exhaust gas A/F ratio on the downstream side of the catalyst (output of the downstream-side exhaust gas sensor) becomes stable, no hunting due to a change in dynamic characteristics of the catalyst occurs, and stable control on the exhaust gas A/F ratio can be performed.
A fourth object of the present invention is to provide a control apparatus for an internal combustion engine, capable of realizing both higher gain (higher response) and stability of a feedback control and also increased robustness.
According to a fourth aspect of the present invention, a control apparatus for an internal combustion engine of the invention calculates a correction amount of an input of a subject to be controlled by proportional derivative control (PD control) in which the gain of a differential term is higher than the gain of a proportional term by proportional derivative means, and regulates the correction amount within a predetermined range by regulating means. Specifically, the invention is characterized in that (i) the correction amount is calculated by the proportional derivative control, (ii) by setting the gain of the differential term to be higher than the gain of the proportional term, the characteristic of start-up of following the target value, of an output of the subject to be controlled is improved, and (iii) the correction amount calculated by the proportional derivative control is regulated within the predetermined range, thereby solving the inconveniences caused by setting the high gain in the differential term (problems of the influence of noise and deterioration in following the target value). Consequently, even to a subject to be controlled having long waste time or a large phase delay and a subject to be controlled having large disturbance, while maintaining the stability of the feedback control, the gain (response) can be increased. Both higher gain and stability in the feedback control can be realized. The control apparatus is not easily influenced by an error in modeling, and robustness can be also enhanced.
A fifth object of the present invention is to provide an exhaust gas concentration control apparatus for an internal combustion engine, capable of properly reflecting a converting state of an exhaust gas passing through a catalyst (A/F ratio of the exhaust gas at the downstream of the catalyst) into exhaust gas A/F ratio feedback control even when the target exhaust gas A/F ratio on the upstream side of the catalyst is deviated from the theoretical exhaust gas A/F ratio range, and having improved catalytic conversion efficiency.
According to a fifth aspect of the present invention, in an exhaust gas A/F ratio control apparatus for an internal combustion engine of the invention, when a sensor for detecting A/F ratio of exhaust gas is provided on each of the upstream and downstream sides of a catalyst, exhaust gas A/F ratio feedback control on the upstream side of the catalyst is performed by exhaust gas A/F ratio feedback control means on the basis of an output of the upstream side sensor, and sub feedback control for reflecting an output of the downstream side sensor into the feedback control on the exhaust gas A/F ratio on the upstream side of the catalyst is performed by sub feedback control means, at least one of parameters of the sub feedback control is variably set by parameter varying means in accordance with a deviation between the exhaust gas A/F ratio on the upstream side of the catalyst and a theoretical exhaust gas A/F ratio. Consequently, also in the case where the deviation between the exhaust gas A/F ratio on the upstream side of the catalyst and the theoretical exhaust gas A/F ratio is large (in a region where the sub feedback control is inhibited in a conventional system), the sub feedback control is executed so as not to excessively correct the deviation. The conversion state of the exhaust gas passing the catalyst (exhaust gas A/F ratio on the downstream side of the catalyst) can be properly reflected in the exhaust gas A/F ratio feedback control on the upstream side of the catalyst. Thus, the catalytic conversion efficiency can be improved as compared with the conventional system.